1. Field of the Invention
The invention relates to a method and system for receiving an ultra-wideband signal with a self-adapting number of propagation paths.
2. Description of the Related Art
The technique of radio communications using an ultra-wideband signal, also referred to as UWB in English, does not use a carrier frequency. Instead of modulating a signal or supporting carrier wave, the information to be transmitted is transmitted directly in baseband, using support pulses having a very short duration of less than one nanosecond, and therefore a very large bandwidth of several GHz.
Since these pulses are transmitted at low power, the spectral density of the power of the transmitted signal is consequently very low.
A UWB signal is thus not a continuous signal, but instead a sequence of very brief pulses having a very low cyclical ratio.
Multiple access to the transmission by a signal of this type is conventionally carried out by means of time leaps (Time Hopping) controlled by a pseudo random sequence. The signal may be modulated in terms of amplitude by the form factor or even the delay of the successive pulses.
In contrast with the concepts of the basic techniques which use a carrier wave, the techniques for transmitting and receiving UWB signals are the only techniques of this kind and are similar to the techniques for detecting spectrum spreading signals.
In particular, the “comb-type” receivers for UWB signals are designed to operate in environments with interference in which the topology of the location in which they are used produces complex transmission channels which are variable or slowly variable owing to numerous multiple secondary propagation paths, and prevents, in practice, the existence of a propagation path which is directly visible.
To this end, the receivers for UWB signals known from the prior art thus conventionally have a structure referred to as being of the “comb-type” which is derived from those used for the receivers of spectrum spreading signals.
As illustrated in FIG. 1A, the UWB receivers mentioned above comprise a branch for receiving via a comb “member”, each receiving branch processing a given receiving path. The outlet of each of the receiving branches is recombined, after weighting, α1, αj, αN, in accordance with the strategy pursued by the designer of the receiver.
In order to ensure correct operation of the receiver, it is necessary to allocate one of the receiving branches for searching for new secondary and/or principal propagation paths for pulses. For a “comb-type” UWB receiver having “N” members or paths, it is therefore necessary to provide N+1 receiving branches.
As also shown in FIG. 1B, in the case of a comb-type UWB receiver, a receiving branch is constituted by an analogue correlator, a correlation pattern generator and an analogue integrator. The tracking of the path relative to the receiving branch in question is brought about by the control logic of the receiver.
When the comb-type UWB receiver is synchronised, the control logic of the receiver brings about the production of a pattern corresponding to the arrival times of a pulse. That produces a correlation pattern configured in order to have a high intercorrelation value with the pulse received and an intercorrelation value of zero in the presence of white noise. A high intermediate intercorrelation value indicates the presence of a direct or secondary pulse.
FIG. 1C illustrates, by way of example, an example of this principle in the case of a 2-PPM digital modulation, the transmission of binary values 0 and 1 being illustrated by the transmission of two pulses A and B which are staggered over time.
The correlation pattern is configured so that the value of the intercorrelation coefficient is positive in the presence of a non-staggered pulse (A) corresponding to the transmission of a zero value, but negative in the presence of a staggered pulse (B) corresponding to the transmission of a value one, and zero when there are no pulses. The correlation pattern is thus symmetrical relative to a centre of symmetry.
However, since a symbol is most often coded over several pulses, it is necessary to integrate the values obtained for the intercorrelation coefficient for each pulse relative to the same symbol and, in this manner, to obtain a global correlation coefficient value for the symbol. This value is transmitted to the control logic of the receiver in order to be interpreted at that location in accordance with the coding method used and thus to find the symbol transmitted.
Another specific example illustrated in the case of a PPM modulation with two simultaneous users each having a pseudo random sequence is illustrated in FIG. 1D. In this example, the symbol is repeated three times, each user therefore transmitting three pulses which represent the same symbol. Consequently, the symbol time Ts is divided into three frames Tf in which each user codes a single unique pulse.
The location of this pulse in the frame Tf is fixed relative to elemental frame intervals by the value of the pseudo random sequence belonging to each user. Finally, each pulse is staggered by a length of time δ relative to the start of each elemental frame interval when the binary transmission illustrated is that of the value 1 instead of the value 0, when there is no staggering operation.
The comb-type UWB receivers mentioned above involve an excessively high level of complexity since each additional member presupposes the integration of an additional receiving branch. Consequently, there is a relatively strict limit on the number of members that a receiver of this type can have in view of the constraints in terms of integration, spatial requirement, cost and consumption.
In practice, it is rare to be able to have a comb-type UWB receiver having more than 4 members, that is five operational receiving branches.
Receivers of this type are therefore limited to high-level uses in which the criterion of cost is secondary compared with that of overall effectiveness in terms of connection quality.
Comb-type UWB receiver solutions which are completely digital have been envisaged, in which the signal received is directly digitised at the output of the antenna. Although, owing to the purely software-based processing of the above-mentioned digitised signal, the structure of receivers of this type no longer corresponds to the traditional comb-type architecture, solutions of this type are not currently viable since current analogue/digital converters are not suitable for a use of this type, and the digital processing operations which are to be carried out on the above-mentioned digital signals cannot be carried out in real time by the current digital signal processors.